I'm jealous of kangaroos. Kangaroos don't have to worry about voracious placentas trying to hijack the rest of the blood supply like women do. For periods, kangaroos can just reabsorb their uterine lining, instead of discarding it all away.
For pregnancy, the baby is born at around the size of the jelly bean. The fetus doesn't grow much in the womb much at all, nor does it squish all the organs internally. Instead, after around 1 month of pregnancy, the baby is easily birthed, crawls out of the kangaroo's vagina, crawls up and into the pouch, latches onto a teat (teats are in the pouch), and grows big while sitting in the pouch.
Unlike birds, they don't even have to worry about pushing out a fat egg.
imagine that. while not pregnant, free pouch to use. pregnancy barely affects the body, non life-threatening, and the joey occupies the external pouch until it can hop by itself. Threatened by something? Some female kangaroos have been documented throwing the joey out of their pouch, at the predator.
The only downside is that the joey poops and pees in the pouch, so it would require regular cleaning